Hydraulic piston cylinder assemblies are often equipped with a closed loop system controlling the position, the pressure, the force or the speed of the unit. Accordingly the piston stroke for example is determined by a distance transducer and this actual quantity as measured and a desired value are delivered to an electronic controller generating an error signal from both quantities and generating therefrom a positioning variable for controlling a valve controlling the fluid path between the power unit, a fluid pressure source and a reservoir.
The quality of the control system can be improved by feeding back certain output variables in a closed loop.
For example, output variables such as the speed and the acceleration of the power unit are fed back for improving a position control.
The output variables are sensed or, respectively are calculated in using a model generator. Due to the quality of the distance signals a differentiation does usually not result in any improvement.
In piston cylinder assemblies having one-sided or two-sided piston rods the differential pressure in both cylinder chambers can be used for an output variable control.
However, certain operative conditions of hydraulic power units cannot be appropriately controlled by merely sensing the distance, the speed, the acceleration or sensing the differential pressure. In the return stroke of a large press, for example, the open valve for decompressing and returning the fluid displaced from the cylinder space to the reservoir may cause severe vibrations, in particular in elongate pipe lines or, respectively in large fluid volumina which vibrations cannot be counterarted by any known speed or acceleration feedback means since the output variables required are scarcely to be measured. This is particularly true for the output variable control of low frequency hydraulic power units including plunger cylinders which are not equipped with an appropriate distance sensing system. A further application is noise reduction of hydraulic power units, for example in stamping and nibbling machines. Here the noise primarily is generated the tool breaks through the material. Breaking through under load results in a high acceleration of the piston resulting in loud noise.